1. Field of the Invention
The present disclosure relates to a thin film transistor substrate for flat panel display device including oxide semiconductor. Specifically, the present disclosure relates to a thin film transistor substrate having the n-type oxide semiconductor material and the stability for the ultra violet light is enhanced.
2. Discussion of the Related Art
Nowadays, as the data process devices such as personal computer, PDA (Personal Digital Assistant), SMART phone, cellular or mobile phone and so on are developed, the requirement for the light weight, thin and small display device is increasing and the various flat panel display devices are developed for satisfying these market trends. The flat panel display devices include the liquid crystal display device (or ‘LCD’), the field emission display (or ‘FED’), the plasma display panel (or ‘PDP’), the electro luminescence device (or ‘ELD’) and so on. For the mass production technology, driving easiness, high definition video quality and low power consumption, the LCD or OLED (organic light emitting diode display device) in which the thin film transistors are arrayed in matrix type is mostly used.
An active matrix type liquid crystal display device (or “AMLCD”) represents video data using the thin film transistor (or “TFT”) as the switching element. The active matrix type thin film transistor substrate drives the pixels of the display device using the amorphous silicon thin film transistor (or “a-Si TFT”). As the a-Si TFT is manufactured with the low cost and under the low temperature processing, it is mainly used for manufacturing the thin film transistor substrate of the flat panel display device.
However, as the a-Si TFT has the low mobility property and bad static electric characteristics, when it is applied to the large area display device, it is hard to ensure good display quality. In order to overcome this problem, the poly silicon thin film transistor (or “p-Si TFT”) is suggested to use for the large area display device. However, there are some other problems. For example, the cost for manufacturing the poly silicon TFT substrate is very high. The manufacturing processes are conducted in high temperature. Furthermore, for the large area display device, it is hard to get the uniform characteristics of the poly silicon TFTs. In that case, the static electric characteristics also degraded.
To overcome these problems, the oxide thin film transistor is suggested. The oxide thin film transistor is not only manufactured under the low temperature processings but also has better static electric characteristics than that of the a-Si TFT or p-Si TFT. Therefore, it is possible to manufacture the thin film transistor substrate having uniform characteristics over the large display area with low cost.
For the oxide semiconductor materials, there is the amorphous indium gallium zinc oxide (a-InGaZnO4: a-IGZO). As the a-IGZO TFT has superior characteristics when using the conventional manufacturing facilities for the a-Si TFT, additional cost is not required to develop the facilities for manufacturing the a-IGZO TFT. For example, the sputter facility can be used to deposit the a-IGZO layer. As for the structure of the thin film transistor having this oxide semiconductor, the back channel etch type and the etch stopper type are considerable. Especially, for better characteristics, the etch stopper type is the best structure for oxide TFT.
When using the a-IGZO semiconductor material, there is no special problem at estimation for the bias temperature stress (BTS) characteristics or photo characteristics, respectively. However, when satisfying the both characteristics simultaneously, the stability of the oxide TFT is seriously degraded. This is the important obstacle when the oxide semiconductor is applied to the display device in which the transparent property is mostly considered.